The present invention relates to selecting an amount of write precompensation in a magnetic storage system. In particular, the present invention measures the time between a location of a data pulse in a window and a following edge of that window during readback from the storage system.
In a magnetic storage (disc drive) system, digital information is magnetically stored upon a magnetic medium on the surface of a rotating magnetic disc. The digital information is represented by selectively polarizing the magnetic field of consecutive areas across the surface of the rotating magnetic disc. When this information is read back from the disc, the magnetic polarization of the medium is sensed and converted into an electrical output signal. This reading operation is done through the magnetic read/write head, which is suspended over the surface of the rotating disc and which provides a raw data signal. The raw data signal is representative of the relative strength of the magnetic flux density present on the magnetic disc.
The raw data signal comprises a series of data "peaks" whose location is used to represent digital information. With high data storage densities, the precise location of a data peak becomes critical because adjacent data peaks are spaced very close together. If a data peak is shifted slightly in time, incorrect information may be read back.
In recording on a magnetic storage system, magnetic dipoles contained in the disc medium are moved past a read/write head. The read/write head consists of an electromagnet with highly focused fringing fields. The magnetic field due to the read/write head current aligns the dipoles in one direction or another representing digital bits. However, the magnetic field of the read/write head extends somewhat over several bit locations. This over-extension of the magnetic field causes adjacent data peaks to be shifted from their respective center positions. Shifting due to the interaction of neighboring bits is referred to as "peak shift." To eliminate the effects of peak shift on subsequent data recovery, the write data is typically precompensated during the recording process. This is done by advancing or delaying the write signal depending on the data pattern and the track radius. This process is referred to as "write precompensation" and is described in U.S. Pat. No. 4,809,088 incorporated herein by reference.
During recovery of the recorded data signal, a data window signal is also generated. The data window signal is generated based upon the data signal using a phase-locked loop. Under ideal conditions, the data window signal is in-phase with the data signal itself, such that each data pulse is located at the center of the window. However, data pulses tend to move away from the center of the data window due to interference from adjacent pulses, also known as peak shifting.
A common technique employed to determine the required precompensation is window margining analysis. This process continually measures the average error rate as the window width and write precompensation are varied. If one measures timing errors with respect to different window sizes, precompensation for the appropriate window size can be selected to provide the lowest error rate. However, this process will not measure the exact spacing of each data pulse within the window. This process confirms whether the data pulse has been moved into the window, but does not provide information on where the data pulse lies within the window.
Alternatively, complex test equipment can measure the individual distances between data pulses and the center of a window. However, the art lacks a simple and accurate method and apparatus for measuring the spacing between a location of a data pulse in a window and a following edge of that window.